You can only pre-register for
DAS as early as 30 days prior to the first park day, and you must have tickets to do that. You could contact Disability Services to discuss your concerns, but they can't pre-qualify him for a DAS because only Guest Relations can issue DAS. Here is the contact info:
For more information, please contact Disability Services at (407) 560-2547 or email
disability.services@disneyparks.com.
Also the webpage for
Guests with Disabilities which has a link to DAS information and other disability-related info as well.